This invention relates to a device for detecting an actual air/fuel ratio of an air-fuel mixture subjected to combustion in a combustor, such as the combustion chambers of an internal combustion engine, based on the magnitude of an oxygen partial pressure in the combustion gas exhausted from the combustor.
In recent automobiles, one of the popularized methods of sufficiently reducing the emission of HC, CO and NOx is to use a three-way catalyst which catalyzes both reduction of NOx and oxidation of HC and CO and an electronically controlled fuel injection system to minutely control the air/fuel ratio so as to maintain a specific air/fuel ratio at which the three-way catalyst exhibits the highest conversion efficiency, and in many cases it is intended to maintain a stoichiometric air/fuel ratio, that is, about 14.5 in gasoline engines. In this method it is usual to perform closed-loop control of the air/fuel ratio by the use of an oxygen sensor installed in the exhaust system to detect a change in the concentration of oxygen in the exhaust gas as an indication of a change in the air/fuel ratio of an air-fuel mixture actually supplied to the engine, because it is more practical to provide an oxygen sensor in the exhaust system than in the intake system of the engine.
An oxygen sensor prevailing for this purpose is of the concentration cell type having a layer of an oxygen ion conductive solid electrolyte, such as zirconia stabilized with calcia, a measurement electrode layer porously formed on one side of the solid electrolyte layer and a reference electrode layer formed on the other side. This oxygen sensor is designed such that the reference electrode layer is exposed to air while the measurement electrode layer is exposed to an exhaust gas and generates an electromotive force the magnitude of which depends on the difference between a reference oxygen partial pressure in air and an oxygen partial pressure in the exhaust gas. When the air/fuel ratio of a mixture supplied to the engine changes across a stoichiometric air/fuel ratio, a great and sharp change is exhibited in the magnitude of the electromotive force which the sensor generates. Accordingly this type of oxygen sensor is suitable for application to engines to be operated with a stoichiometric or approximately stoichiometric air-fuel mixture. From an industrial point of view, however, this type of oxygen sensor is rather low in productivity and difficult to achieve desirable size reduction because of design restrictions placed on it by the necessity of introducing air to the reference electrode layer.
An advanced oxygen sensing device of the concentration cell type is proposed in U.S. patent application Ser. No. 12,763 filed Feb. 16, 1979, now U.S. Pat. No. 4,207,159. This device has a gas permeable porous layer of a solid electrolyte, a porous and film-like measurement electrode layer on one side of the solid electrolyte layer, a reference electrode layer on the other side and a shield layer formed so as to cover the reference electrode layer entirely. The two electrode layers are usually made of platinum, and, as a primary feature of this device, a DC power supply is connected to the two electrode layers to force an electric current to flow through the solid electrolyte layer between the two electrode layers while the measurement electrode layer is exposed to an exhaust gas. The flow of the current between the two electrode layers causes migration of oxygen ions through the solid electrolyte layer and proceeding of electrolytic reactions between oxygen ions and oxygen molecules at the surfaces of the respective electrode layers, and as a result a reference oxygen partial pressure is established at the interface between the reference electrode layer and the solid electrolyte layer. An electromotive force measured between the reference and measurement electrode layers of this device exhibits a sharp change in its magnitude when the air/fuel ratio of an air-fuel mixture from which the exhaust gas is produced changes across the stoichiometric ratio. (A more detailed description about the function of this device will be given hereinafter.) Accordingly, this device is useful for engines to be operated with a stoichiometric air-fuel mixture and advantageous in that there is no need of using an external oxygen source to provide a reference oxygen partial pressure and that the device can be made small in size and produced easily.
Meanwhile, the development of so-called lean-burn engines has been in progress with the view of attaining a maximal thermal efficiency. Also, so-called rich-burn engines have attracted attention because of the possibility of achieving a very high mechanical efficiency and have already been put into practice when recirculation of exhaust gas is employed as a measure of decreasing the emission of NOx. Accordingly there is a demand for an oxygen sensor which is to be used in exhaust gases and enables detection not only of a stoichiometric air/fuel ratio but also air/fuel ratios either above or below the stoichiometric ratio.
According to U.S. patent application Ser. No. 28,747 filed Apr. 10, 1979, now U.S. Pat. No. 4,224,113, it is possible to detect air/fuel ratio values of either a lean mixture or a rich mixture supplied to a combustion engine by using the above described device of U.S. Pat. No. 4,207,159 and by adequately determining the intensity of the electric current forced to flow through the solid electrolyte layer. More particularly, when the current is made to flow from the measurement electrode layer towards the reference electrode layer of the device and the current intensity is below a certain critical value, the output voltage of the device in the exhaust gas remains negligibly low while a rich mixture is supplied to the engine but abruptly rises to a maximal level when the air/fuel ratio reaches the stoichiomeric ratio and, when a lean mixture is supplied to the engine, exhibits a gradual lowering as the air/fuel ratio supplied to the engine becomes higher. Accordingly it is possible to detect a stoichiometric air/fuel ratio and higher air/fuel ratios by using the device in this manner. When a current of an intensity below a certain critical value is made to flow in the reverse direction, the output voltage of the device remains negligibly low while a lean mixture is supplied to the engine, abruptly rises to a maximal level at the stoichiometric air/fuel ratio and, when a rich mixture is supplied to the engine, exhibits a gradual lowering as the air/fuel ratio decreases from the stoichiometric ratio. In this case, therefore, a stoichiometric air/fuel ratio and lower air/fuel ratios can be detected by this device.
However, the air/fuel ratio detection method of U.S. Pat. No. 4,224,113 is inconvenient in that an output voltage value corresponding to a certain air/fuel ratio value of a lean mixture (or a rich mixture) appears also when the output voltage undergoes a sharp change upon arrival of the air/fuel ratio at the stoichiometric ratio. Accordingly a closed-loop air/fuel ratio control system based on this method needs to include certain means for judging whether a measured value of the output voltage indicates the stoichiometric air/fuel ratio or a higher (or lower) air/fuel ratio or means for taking out only output voltage values in the inclined portion of the (air/fuel ratio)-to-(output voltage) characteristic curve. Of course the need for the provision of such means results in undersirable complication of the control system.